1. Field of the Invention
The present invention relates to memory devices, and more particularly to nonvolatile memory devices having single-bit and multi-bit storage regions.
2. Description of the Related Art
Market forces have dictated a gradual demand increase for high-capacity memory devices that don't require periodic refresh. A NAND flash memory device is a nonvolatile memory device which provides the large capacity, high integrity and ease of use demanded by consumers. Since NAND flash memories retain data even if a power is turned off, NAND flash memory devices are well-suited for certain electronic devices, such as mobile terminals, mobile computers, and so forth, where a power supply can be abruptly interrupted.
As is well known in the flash-memory semiconductor industry, the data-retention characteristics and the number of program/erase cycles of flash memories are primarily related to the reliability of the floating gate structure that is the basis of the underlying technology. Various failure mechanisms include the lowering or raising of a cell transistor's threshold voltage through the errant release or accumulation of charges (electrons). Further, the repetition of program/erase cycles can stress the tunnel oxide layer at the bottom of a cell transistor's floating gate, which can result in the eventual destruction of the tunnel oxide layer. Additional problems can also arise as charges are trapped in the tunnel oxide layer during program/erase operations, which can change the threshold voltage windows and the program/erase times of the memory device.
Flash memory devices can store single-bit data or multi-bit data depending on how their memory cells control and respond to threshold voltages. For instance, single-bit data can represented by two voltage ranges separated by a threshold voltage with the two voltage ranges corresponding respectively to data ‘1’ and data ‘0’, whereas 2-bit data can be represented by four voltage ranges separated by three threshold voltage levels with the four voltage ranges corresponding respectively to data ‘11’, data ‘10’, data ‘00’, and data ‘01’.
In general practice, the reliability of multi-bit flash memory devices is less than that of single-bit flash memory devices as multi-bit flash memory devices tend to be more sensitive to the above-mentioned characteristics influencing reliability. For this reason, multi-bit flash memory devices are used for simple data storage while critical or secure data is stored in single-bit flash memory devices (or some other high-reliability memory). Unfortunately, this practice can increase systems memory costs. Accordingly, new technology related to flash memories is desirable.